1. Field of the Invention
This invention relates to solid electrolytic capacitors of the type which make use of a conductive polymer film as a solid electrolyte. The solid electrolytic capacitors have significantly improved life characteristics when used under high temperature anti high humidity conditions. The invention also relates to a method for manufacturing the capacitors of the type mentioned above.
2. Description of the Prior Art
Recent trends toward digitalizatlon and miniaturization of circuits used in the electric and electronic fields have demanded capacitors which are small in size and large in capacitance with a low impedance in a high frequency range.
Known capacitors which have been used in high frequency ranges include, for example, plastic film capacitors, mica capacitors, layer-built ceramic capacitors and the like. These capacitors are disadvantageously so large in size that a large capacitance is difficult to obtain.
On the other hand, a certain type of electrolytic capacitor is known as having a large capacitance. This type of capacitor includes, for example, an aluminum dry electrolytic capacitor and an aluminum or tantalum solid electrolytic capacitor., With the aluminum dry electrolytic capacitor, anodic and cathodic aluminum foils which have been preliminarily etched are convolutely wound through a paper separator. A liquid electrolyte is impregnated in the separator. With aluminum or tantalum solid electrolytic capacitors, solid electrolytes are used in order to improve the characteristics of the electrolytic capacitors. For the formation of the solid electrolyte, an anode foil is immersed in a manganese dioxide solution, which is thermally decomposed in a high temperature furnace at approximately 350.degree. C. to form a manganese dioxide layer on the foil. Since the solid electrolyte is used, there are produced no disadvantages of liquid electrolytic capacitors such as of volatility of liquid electrolytes and the lowering of performance of the capacitor as will be caused by coagulation of liquid electrolytes at low temperatures. Accordingly, such solid electrolytic capacitors are better in frequency and temperature characteristics than liquid electrolytic capacitors. Like tantalum electrolytic capacitors, the aluminum electrolytic capacitor is advantageous in that since an anodized film serving as a dielectric layer can be formed very thinly, a large capacitance can be realized.
Recently, solid electrolytic capacitors have been developed using organic semiconductors such as 7,7,8,8-tetracyanoquinodimethane (TCNQ) salts as the solid electrolyte (Japanese Laid-open Patent Application No. 58-17609). Moreover, another type of solid electrolytic capacitor has also been proposed. In the capacitor, at least one of electrodes of the capacitor is made of a conductive polymer which is formed by electrolytic polymerization of polymerizable monomers such as pyrrole, thiophene, furan and the like. More particularly, a solid electrolytic capacitor has been reported in which a polyimide thin film serving as a dielectric film is electrodeposited on the surface of an electrode. A chemically polymerized conductive polymer film is built up on the polyimide thin film, on which an electrolytically polymerized conductive polymer film is further built up, thereby obtaining the capacitor (Electrochemical Society 58th Meeting Technical Digest of Papers, 1991 (Spring), pp. 251 and 252).
A diversity of capacitors have been thus employed in the electric and electronic fields. These capacitors are not necessarily satisfactory with respect to characteristic properties. For instance, film and mica capacitors are so large in size that a large capacitance is difficult to attain. The builtup ceramic capacitors have been developed to satisfy the requirements for the small size and the large capacitance but are disadvantageous in their high production costs and poor temperature characteristics.
With the aluminum electrolytic capacitors, the oxide film is liable to undergo damages. This makes it necessary to provide an electrolyte between the oxide film and a cathode in order to repair the damages. Where the electrolyte used is liquid, the resultant capacitor suffers leakage of the electrolyte. In addition, the liquid electrolyte is ion conductive. For these reasons, the capacitor brings about a decrease of electrostatic capacitance and an increase of loss, with poor high frequency characteristics and a great loss in low temperature ranges.
With solid electrolytic capacitors using manganese dioxide, the thermal decomposition at high temperatures has to be repeated several times, resulting in the damage of the oxide film. Additionally, manganese dioxide has a high specific resistance. For these reasons, the loss of the resulting capacitor is not sufficiently small in a high frequency range.
On the other hand, solid electrolytic capacitors using organic semiconductors such as TCNQ salts have better high frequency characteristics than those using manganese dioxide. However, on application of an organic semiconductor, the specific resistance is undesirably increased and the organic semiconductor is relatively poor in adhesion to the anode foil. Thus, such capacitors do not necessarily exhibit ideal characteristics.
Moreover, when a conductive polymer thin film is used as an electrode of capacitors, the capacitor exhibits good frequency and temperature characteristics along with good life characteristics under high temperature conditions. However, the dielectric film is liable to be attacked with moisture, so that the capacitor has not good life characteristics under high temperature and high humidity conditions.